Long‐term trends in, and spatial variation of, solar irradiances in Ireland

Analysis of the pooled data of global K ↓, and diffuse D , irradiances measured at eight sites in Ireland, with time series varying from 14 to 42 years showed significant decreases averaging 16.39 MJ m −2 (0.62%) year −1 in K ↓ and 3.10 MJ m −2 (−0.77%) year −1 in D . The long‐term trends differed markedly between the eight pyranometer stations. No significant changes were found at the four northern stations. Significant increases were found at Dublin airport near the east coast, highly significant decreases were found at the three stations in the south of the country, including Valentia Observatory in the extreme south‐west, whose measurements extended over 40 years. At Valentia, the decreases in K ↓ and D , equivalent to −0.43% and −0.42% per year, respectively, were accompanied by significant decreases in radiation balance and hours of bright sunshine, −0.86% and −0.32% per year, respectively. Although no large or significant trends in daytime cloud cover were observed at Valentia there was a significant, negative interaction between K ↓and the small, random interannual variations in cloud cover; the rate of decline in K ↓ was three times larger in years of below‐average cloudiness than in years of above average cloud cover. North–south gradients in K ↓ and D were significant for both mean annual and mid‐winter irradiances; the values measured at Dublin airport were anomalously high but appeared to be site‐specific. Possible causes of the changes in irradiances found, and their agricultural, hydrological and climatological consequences are briefly discussed. © 1998 Royal Meteorological Society